Polymer clay is perfect for buttons! I like to use cane slices because you automatically end up with multiple matching buttons. The easiest way is to cut a generous slice and form holes with a tooth pick. These are from my snowflake tutorial.
However, if you want shank buttons, you can always add those little rings that you find in the jewelry section of the craft store. You wouldn’t think they would hold so strong, but as long as you add or spread the clay to seal the ring in, it won’t come out. Well, mine haven’t so far! The only problem was that when I squished the ring in, the pattern from the cane got distorted. I decided to make and bake the blank buttons first. After the blanks were baked, I added a thin slice from a cane and baked it again.
Quite a few of the cane tutorials in my etsy shop would make lovely custom buttons.